Change Over Time

Change Over Time

Around 1500, the southwestern ponderosa pine ecosystem extended over thousands of square miles, and while the current distribution is similar to that historic extent, the overall character or “ecological condition” is vastly different. Human activities in recent centuries have influenced this ecosystem and altered its ecological integrity throughout much of the range.

These man-made influences on the ecosystem include logging, livestock grazing, and wildfire suppression. Timber harvests have reduced the number of large, mature ponderosa trees in many areas. Meanwhile, overgrazing can reduce woodland grasses and forbs, exposing bare soil where ponderosa seedlings germinate and establish dense young stands of pine trees under the older mature pines. 

Natural fire cycles have been altered in several ways – sometimes hastened, sometimes suppress – and, ironically, the growing fragmentation of these woodlands occasioned by expanding suburban and rural settlements has increased both the importance and the difficulty of fire prevention in recent decades.

During the mining era, fires were deliberately set in order to reveal locations of mineral deposits. Such activity destroyed many of Colorado’s old-growth stands in the early 1900s. The suppression of wildfires on other sites over the past 150 years has allowed other tree species to become established under the pines: pinyon pine (Pinus edulis) at lower elevations, and white fir (Abies concolor) or Douglas-fir at higher elevations.  These other trees are gradually replacing the ponderosa pine in some locations.

Fire suppression also builds up available stocks of fuel in the form of fallen branches and needle litter. In such conditions, these accumulated fuels and dense growths of younger trees can feed hotter, more catastrophic fires into the crowns of the taller trees, killing most of the pines. Dense stands are also more susceptible to insect attacks, because as competion for sunlight, water and nutrients increases, individual trees are less healthy and insect infestations can become more widespread and severe, causing increased die-off.

The sum of these human effects has led to a major decline in old-growth ponderosa pine woodland across the landscape – perhaps by as much as an 80% reduction. Many of the Southwest’s ponderosa pine savannas and woodlands have grown into dense forest, diminishing both the abundance and diversity of grasses and flowering plants. These greatly simplified woodlands, with dense stands that cover large areas and lack large, old trees, are no longer as patchy and no longer provide the same complex vegetation structure for wildlife habitat. Many bird, invertebrate, and mammal species rely on the variable sizes and ages of woodlands, conditions that increasingly rare in southwestern Ponderosa Pine ecosystems. Signifying these changes, a June 2002 fire in Colorado consumed over 130,000 acres, much of it ponderosa pine woodland. In this dry landscape, severely burned woodlands often persist as grasslands or shrublands and may take several hundreds of years to regain their earlier character.

Prognosis for the Future

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